1. Field of the Invention
The present invention relates to an optical pickup and an optical disc device, and is suitable for application to an optical disc device compliant with an optical disc having plural recording layers, for example.
2. Background Art
In related art, optical disc devices that apply a light beam is applied to an optical disc such as a CD (compact disc), a DVD (Digital Versatile Disc), and a Blu-ray Disc (registered trademark, called BD as below) and read the reflected light thereof for reproduction of information have been widely spread.
In the optical disc, information to be recorded is coded and modulated, and recorded as pits or the like on helically or coaxially formed tracks.
Accordingly, in the optical disc device, the light beam is collected using an objective lens, and the light beam is focused on the tracks helically or coaxially formed on the recording layer of the optical disc when the information is reproduced from the optical disc.
The optical disc device receives the reflected light by providing a light receiving area having a predetermined shape in a photodetector. The optical disc device calculates a focus error signal and a tracking error signal representing amounts of shift between the track to which the light beam should be applied (hereinafter, referred to as a target track) and the focal point of the light beam with respect to the focus direction and the tracking direction based on the light reception result, respectively.
Subsequently, the optical disc device moves the objective lens in the focus direction based on the focus error signal and moves the objective lens in the tracking direction based on the tracking error signal, and thereby, focuses the light beam on the target track.
Some optical discs have two recording layers and reflect the light beam on the respective recording layers at respective predetermined reflectances. For the optical disc, the optical disc device is arranged to make control of focusing the light beam on a desired recording layer and detect the reflected light thereof.
Here, in the optical disc device, interlayer stray light formed by reflection of a part of the light beam on the other recording layer than the desired recording layer may be applied to the light receiving area of the photodetector. In this regard, there is a possibility that the optical disc device may not accurately perform tracking control because an error occurs in the tracking error signal due to the interlayer stray light.
Further, regarding the optical disc, it is conceivable that four or six recording layers, for example, are provided to increase the recording capacity. It is considered that, in such an optical disc, the layer distance between the closest (i.e., adjacent) recording layers may be smaller and the layer distance between the farthest recording layers may be larger compared to the layer distance of the optical disc having two recording layers in the past.
Accordingly, in the optical disc device, application conditions of the interlayer stray light may differ according to the interlayer distance between the desired recording layer and the other recording layer.
On this account, an optical disc device using a so-called one-beam push-pull method is proposed for avoiding the influence by interlayer stray light even when the number of recording layers is increased in an optical disc (e.g., see JP-A-2008-135131, FIG. 8).
In the optical disc device, the reflected light is diffracted in different directions (shown by arrows in the drawing) with respect to each area of reflected light using a hologram element 1 and the like as shown in FIG. 1, and the reflected light is received with respect to each area of reflected light by plural light receiving areas provided in a photodetector 2 as shown in FIG. 2.
As shown in FIGS. 1 and 2, the optical disc device diffracts the reflected light in the area containing a so-called push-pull component (main area) and the area containing a so-called lens-shift component (sub-area) of the reflected light in different directions.
Thereby, the optical disc device is adapted to prevent the application of the interlayer stray light to the light receiving area (shown by broken lines in the drawing) in the case of using any recording layer as the desired recording layer, or prevent occurrence of errors in the tracking error signal and the like even when the light is applied.